Computer technology requires memories having large storage capacity and high speed. Typically, in a modern computer, a semiconductor memory is employed as high-speed primary memory and magnetic disks are used for a large volume secondary memory.
Prior to the development of semiconductor memories, the high-speed primary memory was implemented using a magnetic core memory. A magnetic core memory comprises a matrix of ring-shaped ferromagnetic cores. Each memory cell of the magnetic core memory includes a ferromagnetic core having two or more wires passing through the center of the core and a sensing coil installed around the core.
When a current I is applied to a wire that passes through the core, a magnetic field is produced which has a magnetic field strength H which is a function of the current I. The magnetic field produced by the current causes a permanent magnetization of the core which is measured by the magnetic induction B. The relationship between B and H has substantial hysteresis with the result that a plot of B versus H, which is known as the magnetization curve or BH loop, is substantially square.
The magnetic induction B in the core has two states, B.sub.r and -B.sub.r, that correspond to the opposite directions of the magnetic field. Accordingly, each core can store a bit of binary data by associating one state with a "1" and the other state with a "0". Illustratively, +B.sub.r may be associated with a binary "1" and -B.sub.r with a binary "0".
The binary data is written into a core memory cell by applying appropriate currents to the wires. If the total current passing through the core is greater than a critical current I.sub.c, the magnetic induction of the core changes from -B.sub.r to +B.sub.r. Similarly, if the current is less than -I.sub.c, the magnetic induction switches from +B.sub.r to -B.sub.r. Advantageously, in an array of magnetic cores, switching is performed as the result of the coincidence of signals on two or more wires. Thus, if the magnetic induction initially has the value of -B.sub.r corresponding to a "0", a binary "1" is stored by applying a current I&gt;I.sub.c /2 to each of the two wires, so that the total current passing through the core is greater than +I.sub.c which causes the magnetic induction to change to +B.sub.r.
The data stored in the core is retrieved by sensing the voltage across the coil induced by switching between the two magnetic states described above. The polarity of the induced voltage indicates the magnetic state of the core prior to switching.
Although the magnetic core memory described above is random accessible and non-volatile, such memory is large, consumes a large amount of power, operates at a slow speed and can not be manufactured to have a high storage density. To overcome these problems, magnetic thin film memory devices have been developed. A magnetic thin film memory consists of a strip of ferromagnetic thin film, two or more wires for writing data formed on the film and a coil around the film for reading data.
In the thin film memory, the magnetic moment M of the film represents the stored information. The magnetic moment M is oriented primarily in the plane of the film, and has two discrete orientations or states M and -M that represent binary "1" and "0". To store a bit of binary data, currents are applied to the wires formed on the thin film. These currents induce a magnetic field that is sufficient for changing the direction of the magnetic moment M. The stored information is retrieved by applying currents to the wires and measuring the induced voltage in the coil. As in magnetic core memory, the currents are typically selected such that a single current has insufficient amplitude to reverse the magnetic moment of the film so that at least two coincident currents are required for storing data.
There are significant drawbacks associated with magnetic thin film memory technology. First, thin film devices have an open magnetic flux structure and therefore the BH loop is smeared by a self-demagnetizing effect. To reduce this effect, the film is typically fabricated as a rectangle whose length is much greater than its width. Since the induced voltage in the coil around the film is proportional to the cross-sectional area of the film, reducing the width of the film also reduces the induced voltage. As a result, the readout signal is easily affected by noise.
Second, in existing magnetic films, the magnetic moment has a preferred in-plane direction. Thus the device is complicated by the necessity of applying currents of different amplitude for storing and retrieving data in the selected orientations. In addition, the thin film devices are not sufficiently small to achieve high densities.
In comparison to magnetic core and thin film memories, semiconductor memory is faster, consumes less power, and can have higher storage densities. Typical semiconductor memories include Dynamic Random Access Memory (DRAM), Static Random Access Memory (SRAM), and Read Only Memory (ROM).
DRAM offers relatively high speed, high density, low power consumption, and is readable and writable. However, both DRAM and SRAM are volatile, that is, they lose the stored information when the power is turned off. In addition, DRAM requires a constant refresh of the stored data which necessitates complex circuitry. While SRAM does not require a refresh, it has high power consumption and does not have high storage density.
ROM's are non-volatile but the information stored in a ROM cannot be updated, i.e., data cannot be easily written into a ROM.
In a typical disk storage system, ferromagnetic material having a substantially square BH loop is coated on the disk; and a magnetic head reads and writes information on the disk as it rotates past the head. The disk is divided into circular tracks. Each track is further divided into small regions in which a magnetic moment has two states that represent binary values. An external magnetic field introduced by the read/write head changes the magnetic moment of each small region so as to store a binary value in the region. Thus, to write data, the magnetic head magnetizes an adjacent small region of the rotating disk material. Stored data is retrieved in the form of a voltage induced in the head by the magnetic moment of the small region as it moves past the head.
Magnetic disk storage systems can store high volumes of data, e.g., 500 Megabytes or more. The magnetic disk storage systems, however, are not random accessible, operate at slow speed due to the requirement of mechanical movement, and require complex mechanical and electronic assemblies.
As will be apparent, none of the above-described memory technologies provides all the features that are desirable in a memory storage system. Thus, there is a present need to develop a non-volatile, high speed, high capacity, random accessible, static, and updatable storage system.